Before a data transmitter and a data receiver can communicate using frequency modulation (FM) techniques, the receiver must establish symbol timing and carrier frequency synchronization with the transmitter. A prevalent technique for accomplishing synchronization includes beginning a transmission with a synchronization signal having a predetermined bit rate and bit pattern. By monitoring the transmission, the receiver can detect the synchronization pattern and perform a correlation with the bit pattern to establish timing synchronization. The receiver also can use the received symbol frequencies to tune precisely to the transmitted carrier.
A requirement of the synchronization signal has been that its timing, carrier frequency, and bit rate should be highly stable relative to the timing, carrier frequency, and bit rate of the data transmitted after the synchronization signal. For fixed base transmitters with ample power supply regulation, stability of the synchronization signal is generally not a problem. For battery-powered portable units, however, maintaining the stability of the synchronization signal can be problematic. For reasons primarily related to small size and light weight, the power supply of a portable unit is typically not as well regulated as that of a fixed base transmitter. When the transmitter of the portable unit begins transmitting, a relatively large power demand is made on the power supply, decreasing the power supply voltage. The decrease in voltage can cause a temporary frequency variation in the oscillators of the portable unit, thereby adversely affecting the stability of the synchronization signal. This phenomenon is sometimes referred to as "load pull."
Thus, what is needed is a method and apparatus in a communication receiver for estimating a symbol timing and a carrier frequency of a received data signal without the need for a highly stable synchronization signal from the portable unit.